A porous polymeric film having a large number of intercommunicable micropores is utilized in various fields such as the separation film to be used to produce ultrapure water, purify chemicals, and treat water; a waterproof moisture-permeable film for use in clothes and sanitary materials; and the separator for batteries.
Secondary batteries repeatingly chargeable and dischargeable are widely used as the power source of OA, FA, and portable devices such as household electric appliances, communication devices, and the like. A lithium-ion secondary battery has a favorable volumetric efficiency when it is mounted on devices and allows the devices to be compact and lightweight. Therefore there is a large increase in the use of the portable devices in which the lithium-ion secondary battery is used.
Owing to research and development of a large secondary battery which have been made to use it in the field of load leveling, a UPS, an electric car and in many fields relating to energetic and environmental problems, the lithium-ion secondary battery which is a kind of a nonaqueous electrolytic solution secondary battery has widely spread in its use because the lithium-ion secondary battery has a large capacitance, a high output, a high voltage, and an excellent long-term storage stability.
The lithium-ion secondary battery is so designed that the upper limit of the working voltage thereof is 4.1V to 4.2V. Because electrolysis occurs in an aqueous solution at such a high voltage, the aqueous solution cannot be used as an electrolytic solution. Therefore a so-called nonaqueous electrolytic solution containing an organic solvent is used as an electrolytic solution capable of withstanding a high voltage.
As a solvent for use in the nonaqueous electrolytic solution, an organic solvent having a high permittivity which allows a large number of lithium ions to be present therein is used. An organic carbonate ester such as polypropylene carbonate and ethylene carbonate is mainly used as the organic solvent having a high permittivity. As a supporting electrolyte serving as a lithium ion source in the solvent, an electrolyte such as lithium phosphate hexafluoride or the like having a high reactivity is used by dissolving it in the solvent.
The separator is interposed between the positive electrode of the lithium-ion secondary battery and its negative electrode to prevent an internal short circuit from occurring. Needless to say, the separator is demanded to have electrical insulating properties as its role. The separator is also required to be stable in an organic electrolytic solution. It is also necessary that the separator has a microporous structure to retain the electrolytic solution and secure a passage to allow lithium ions to move between electrodes in charge and discharge. To satisfy these demands, a porous film containing an insulation material such as polyolefin as its main component is used as the separator. Because the porous structure of the separator greatly affects the output of the lithium-ion secondary battery, the characteristic features of the lithium-ion secondary battery are in discussion by using various parameters for evaluating the porous structure.
The method of producing the separator is classified into two kinds, namely, a wet type method and a dry type method. There is a great difference between the porous structure formed by using the wet type method and that formed by the dry type method.
The wet-type production method includes the step of mixing polyethylene resin and an addition component such as a plasticizer with each other and molding the mixture into a sheet and the step of extracting the addition component with a solvent and thereafter stretching the sheet-shaped mixture or stretching the sheet-shaped mixture and thereafter extracting the addition component with the solvent to form the porous structure. This method is capable of forming a three-dimensional porous structure.
The dry-type production method includes the step of melting and extruding crystalline polyolefin resin, the step of cooling the polyolefin resin at a high draft ratio to solidify it to form a highly anisotropic sheet of the polyolefin resin, and the step of stretching the produced sheet in a mechanical direction to form the porous structure. This method is capable of forming a two-dimensional porous structure long in the mechanical direction.
Various porous structure-controlled separators have been proposed. Descriptions are made in specifications that the characteristic features of batteries are displayed according to the porous structures.
Description is made in the specification of U.S. Pat. No. 4,098,401 (patent document 1) that the cycle characteristic can be improved by decreasing the air permeability of the separator. Description is made in the specification of U.S. Pat. No. 4,220,329 (patent document 2) that the discharge characteristic and output of the separator are evaluated based on the ratio between the amount of permeated moisture and that of permeated air. Description is also made in the specification of the patent document 2 that when the average flow rate diameter pressure of the separator is low, i.e., when the diameter of a pore is large, the cycle characteristic of the separator is excellent. The dry-type separator disclosed in Japanese Patent Application Laid-Open No. 2000-348703 (patent document 3) is composed of the resin having a low melting point to make the air permeability thereof low.